Both stand at opposites in a cultural and political split that has divided millions of U.S. Latinos for decades as both parties vie for pivotal Latino voters.

Although they often are lumped together as Hispanics, Rubio and Castro are emblematic of acute political distinctions between Mexican-Americans, who are the largest Latino group in the U.S., and Cuban-Americans, who are the most politically active. Despite their shared language, these two constituencies have different histories in the United States and are subjected to distinctions in immigration policy that go easier on Cuban immigrants.

They didn't come from Chihuahua or Sonora in Mexico and from poor backgrounds. They came from affluent backgrounds and have a different perspective. The Republican Party also has opened doors just for them.

- Moises Venegas, a retired Mexican-American educator

"Historically, many Cuban-Americans for the last few decades have tended to be a little more conservative. So it's not surprising that you would see Sen. Rubio and the Republican nominee for Senate in Texas, Ted Cruz, running as Republicans," Castro told The Associated Press. "And I don't begrudge them for that. I think the policies they espouse are wrong, are not the best ones. But, you know, they're doing what they believe. And I applaud them for that."

Rubio, 41, was born in Miami. His parents left their native Cuba for the U.S. 2½ years before Fidel Castro overthrew the Cuban government. Fifty-nine percent of Cubans in the U.S. in 2010 were foreign-born, according to the Pew Hispanic Center, and three-quarters were American citizens.

Julian Castro, 37, was born in the U.S., as were his parents. Almost 64 percent of people of Mexican descent in the country are U.S.-born, according to the Pew Hispanic Center.

Moises Venegas, a retired Mexican-American educator and Latino community activist in Albuquerque, N.M., said the two groups have little in common besides an historical connection to Spain, and Spanish surnames.

"The Cubans have never been one of us," Venegas said. "They didn't come from Chihuahua or Sonora in Mexico and from poor backgrounds. They came from affluent backgrounds and have a different perspective. The Republican Party also has opened doors just for them."

Pedro Roig, a Cuban-American attorney and senior researcher at the Institute for Cuban and Cuban-American Studies in Miami, disputed the notion that there is significant rivalry between the groups. He attributes divisions between Cuban- and Mexican-Americans in part to geography and noted that many in the Cuban community admire Castro's selection as the Democrats' keynote speaker.

"Sometimes Cuban-Americans, we have created an enclave in the area of South Florida, which is much more limited than the Mexican settlement in the United States," Roig said.

Of the 52 million Latinos in the U.S., 33 million are of Mexican descent, followed by 4.7 million who are Puerto Rican and 1.9 million of Cuban descent, Pew Hispanic Center numbers show. The remaining 10 largest Latino groups are Salvadorans, 1.8 million; Dominicans, 1.5 million; Guatemalans, 1.1 million; Colombians, 972,000; Hondurans, 731,000; Ecuadorians, 665,000; and Peruvians, 609,000, the center reported.

In 2008, 9.7 million Latino voters cast ballots in the presidential election, and 5.2 million were Mexican-Americans, about 45 percent of eligible Mexican-American voters, according to Pew Hispanic Center data. When it comes to showing up at the polls, however, Cuban-Americans outpace Mexican-Americans — some 713,000 Cuban-Americans showed up to vote in 2008, 69 percent of eligible Cuban-American voters, the center found.

Obama won 47 percent of the Cuban vote in Florida that year, according to data from The Associated Press.

In Texas, some Republican candidates garner roughly 30 percent of the Hispanic vote, which is overwhelmingly Mexican-American, said Antonio Gonzalez, president of the San Antonio-based Southwest Voter Education Project.

Immigration is the greatest source of division between the groups, with Cubans having an easier and faster route to legal residency and citizenship. Early migrations of Cubans included upper- and middle-class families, but people who came to the U.S. during the 1980s Mariel boatlift were not as well-off.

Cuban-Americans began embracing the GOP in the early 1960s after the Bay of Pigs invasion, which failed to topple Fidel Castro. The loyalty deepened after President Ronald Reagan courted Cubans with his anti-Castro policies.

The U.S. amended the Cuban Adjustment Act of 1966, which had provided Cubans in the country with temporary visas and a path to legal citizenship, with what is known as the "wet foot, dry foot policy." The 1995 measure allows Cubans who reach American shores, "dry foot," to apply for legal residency and eventually citizenship. Cubans who are intercepted at sea, "wet foot," are returned to Cuba or sent to another country that will accept them.

By comparison, Congress has for years refused to rewrite immigration laws to provide U.S. residency for immigrants in the country without legal permission, many of whom are from Mexico. It also voted down a bill that would have given residency to immigrants brought to the country by their parents who entered or stayed illegally.

While some Cuban-Americans have hoped for decades for a return to a free Cuba, many Mexican-Americans recognize parts of the U.S. as historically Mexican. "We didn't cross the border, the border crossed us," is a favorite refrain. Mexican immigration has fed much of the U.S. population growth in recent decades.

DeeDee Blase, founder of the Arizona-based Tequila Party, an independent political group made up largely of Mexican-Americans, said Cuban-Americans have failed to support policies important to Mexican-Americans, like immigration reform and health care, while wanting Latinos to rally around the trade embargo on Cuba. Blase is Mexican-American.

Guarione Diaz, outgoing president of Miami-based Cuban-American National Council, said resentments are disappearing as more Mexican-Americans have moved to Miami and more non-Cuban politicians are elected to offices with heavy Cuban support. Intermarriage between the groups has bridged the divides along with growing Latino unity around equal access issues, Gonzalez said.